The present invention relates in general to communication systems and components, and is particularly directed to a transimpedance circuit, that is configured to transform a pair of differentially sensed input currents into a very precise, single ended output voltage. The transimpedance circuit provides a very high degree of common mode rejection, can operate in high or low D.C. voltage and current environments, and consumes very little power. As a non-limiting example, the transimpedance circuit of the invention may be used to sense differential tip and ring currents of a subscriber line interface circuit.
A wide variety of electronic circuit applications require the differential measurement of two (complementary) currents and some prescribed amount of rejection of their common mode components. In some applications, the currents being measured exist in a high D.C. voltage and high D.C. current environment, yet their information content is ultimately to be employed in a low voltage and low current environment, with demanding requirements for accurate amplification and filtering, plus the additional requirement for low idle channel noise. As a non limiting example, various equipments employed by telecommunication service providers employ what are known as xe2x80x98SLIC""s (subscriber line interface circuits), to interface (transmit and receive) telecommunication signals with respect to tip and ring leads of a wireline pair. Since the length of the wireline pair to which a SLIC is connected can be expected to vary from installation to installation, may have a very significant length (e.g., on the order of multiple miles), and transports both substantial DC voltages, as well as AC signals (e.g., voice and/or ringing), it has been difficult to realize a SLIC implementation that has xe2x80x98universalxe2x80x99 use in both legacy and state of the art installations.
In accordance with the present invention, shortcomings of conventional transimpedance circuits, such as, but not limited to those intended for use in telecommunication service providers"" wireline equipments (such as SLICs) that may be installed in a wide range of voltage and current environments, are effectively obviated by a new and improved, transimpedance circuit, that is capable of performing a very precise differential input current to single ended output voltage conversion, while enjoying a very high degree of common mode rejection, and reduced power dissipation, thereby making it particularly suited for remote site subscriber installations.
For a SLIC application, respective (tip and ring associated) current sense resistors may be installed in the closed loop, negative feedback paths of xe2x80x98tipxe2x80x99 and xe2x80x98ringxe2x80x99 path current sense amplifiers. The currents through the sense resistors may contain a desirable differential current component IDIFF and a common mode undesirable component ICOM. The current sense amplifiers provide substantial performance in terms of gain and gain-bandwidth product, so that any voltage dropped across the sense resistors appear as a negligibly small component of the voltage between the tip and ring terminals of the SLIC.
A voltage drop proportional to the current through each of the (tip/ring path) sense resistors is supplied as a differential control voltage to respective differential coupling circuits installed between associated bias current sources and the complementary polarity inputs of an operational amplifier, that provides the single ended output voltage. Each bias current source is coupled through an associated pair of bias resistors for the differential coupling circuits. As will be described, the maximum current that can be sensed in each complementary (tip/ring) current flow path of the transimpedance circuit is limited by the product of a maximum bias current supplied by the respective (tip and ring path) bias current sources and the ratio of a pair of transistor emitter bias resistances to the resistance value of the (tip and ring path) sense resistors.
In addition to differentially sensing the complementary (tip and ring) currents flowing through the sense resistors and their associated differential coupling circuits, the input terminals of the operational amplifier are coupled to a linearity compensator circuit, which is configured to provide sufficient overhead voltages in the presence of worst case voltage swing conditions. The linearity compensator circuit has a differential amplifier configuration, coupled to close a negative feedback loop from the single ended output and one of the inputs to the operational amplifier, relative to a reference voltage balancing path coupled to the amplifier""s other (complementary) input. This balanced coupling configuration forces the corresponding terminals of a pair of load resistors coupled to the input ports of the operational amplifier to the same potential, irrespective of variations in the sensed input currents.
For this purpose, a first differential compensator portion of the linearity compensator includes a first xe2x80x98overhead voltagexe2x80x99 emitter-follower transistor having its collector-emitter path coupled in circuit with a first bias current source. This first overhead transistor has its base coupled to receive a reference voltage established by a voltage drop across a resistor coupled to receive a prescribed overhead bias current. The emitter output of the first overhead transistor provides base drive to a first emitter-follower configured compensator transistor pair, the current output of which is coupled through a first load resistor to the (+) input of the operational amplifier.
To close the negative feedback loop of the operational amplifier, a second compensator portion of the linearity compensator includes a second overhead voltage emitter-follower transistor having its collector-emitter path coupled in circuit with a second bias current source, and its base coupled through a feedback resistor to the single ended voltage output of the operational amplifier. The emitter output of the second overhead transistor provides base drive to a second emitter-follower configured compensator transistor pair, the current output of which is coupled through a second load resistor to the (xe2x88x92) input of the operational amplifier.
By matching the bias resistors for the differential coupling circuits and the load resistors and parameters of the complementary sides of the differentially configured coupling and compensator circuits installed between the sense resistors and the operational amplifier, the single ended output Voltage VOUT produced at the output of the operational amplifier is effectively linearly definable in terms of the sensed current IDIFF and the values RSENSE of the sense resistors, as VOUT=2*IDIFF*RSENSE. BY optimizing the match between the resistance values of the sense resistors, and the match between the resistance values of the load resistors and the resistance values of the bias resistors common mode output error is minimized.